A circuit comprising a plurality of units with semiconducting elements constitutes an integral part of an electric power converter, where they are used as electronic power switches. These switches are arranged in series connection, where each switch is capable of maintaining a part of the voltage applied over the converter. Known power semiconductors are capable of holding a voltage of 1 to 6 kV. By series connection of a plurality of such switches a converter may maintain a voltage within a range of 10 to 500 kV. Each switch comprises a plurality of semiconducting elements that may be connected in series and/or in parallel to achieve a performance of desire. The series connection will increase the voltage capability and the parallel connection will increase the current capacity.
In a voltage source converter (VSC) the electronic power switches comprises semiconductors of the turn-off type. Such converters are used in high voltage direct current (HVDC) applications for converting direct current to alternating current or inversely. Such converters are also used in static var compensators (SVC) and reactive power compensation (RPC) plants for balancing the power transmission within a power network.
Semiconductors like GTO thyristors and IGBT are suitable for high power applications. Semiconductors of the latter kind is often preferable since they combine good power handling ability with properties which make them well suited for connection in series. They may be turned off with high accuracy. In such constellations a plurality of IGBTs form valves in a voltage source converter for handling voltages up to 500 kV.
Among the existing HVDC transmission system containing voltage source converters there is known at least two configurations. One such configuration comprises a two level converter bridge for use preferably up to 65 MW. The two level bridge is the most simple circuit configuration to build up a three phase forced commutated VSC bridge. The bridge consists of six valves each containing a valve unit comprising switching means. Each valve unit is built up of a plurality of series connected turn-off devices and anti parallel diodes.
A second configuration comprises a three level converter bridge for use preferably up to 330 MW. The three level converter bridge comprises in all 18 valve functions. The three level bridge also comprises extra valve units in comparison with the two level bridge.
Although the two level converter has a simple construction it also has a drawback of high operation losses. The three level converter offers a better way of control but has drawbacks of higher semiconductor cost, bulky AC and DC filters and a possible occurrence of a DC unbalance. For a the three level converter in comparison with a the two level converter, there is a cost transfer from operation (high switching losses) to investment (increased number of valves).
A Pulse Width Modulation (PWM) signal is used to control the voltage source converter. When the HVDC system transmits an active effect the voltage and the current is almost in phase. Under such condition an Optimized Pulse Width Modulation (OPWM) method is advantageous. The pulse train of the OPWM is designed to control the fundamental bridge voltage and at the same time optimizing the criteria for controlling the harmonics. The OPWM signal is calculated in advance and supplied to the valve controller. The OPWM signal is constructed such that fewer switching operations occur when the current is high. Since the heat generation of the semiconductors depend on both the voltage level and current level at switching there will be less heat generation in the semiconductors when using the OPWM method.
Semiconductors are sensitive to heat. The valve operates well below the max allowed junction temperature. However, when this temperature is exceeded the semiconductor will malfunction. Therefore most voltage source converters comprise a cooling system for transporting away the heat. Thus, by a cooling system the performance of the semiconductor may be increased. Another way of increasing the performance of the semiconductor is the reduction of heat generated by switching losses. As discussed above one way of accomplish this is to reduce the number of switching events and another way is to arrange the switching events where the current is low. This is obtained by OPWM via fewer switching, i.e., lower switching frequency, and avoiding switching at high current.
The drawback of OPWM is its poor transient control capability. Due to the modulation method it is not an on-line modulation and some switching instants may not occur continuously and regularly. The poor transient control capability, which leads to transient over DC voltage and over AC current, not only increases cost due to over-dimension the apparatus, but also worsen the performance of the transmission system.
To be able to handle the transients caused by AC faults or other disturbances, an on-line modulation method, i.e., carrier based PWM method, is used. The disadvantage of carrier based PWM is that it requires a higher switching frequency. It switches continuously independent of low or high current. As a result it gives high losses. Hence the carrier based PWM method offers a faster dynamic control during a transient behavior. However, since the switching of the semiconductor also occurs when the current is high the carrier based PWM method is more heat generating than the OPWM method.
A high voltage direct current transmission system as shown by the schematic single line and block diagram in FIG. 1 is previously known. A first and a second converter station STN1 and STN2 respectively are connected to each other via a direct current link having two pole conductors W1 and W2 respectively. Typically, the pole conductors comprise cables but may also, at least in part, comprise overhead lines. Each converter station contains a capacitor equipment, C1 and C2 respectively, connected between the pole conductors, and a voltage source converter, CON1 and CON2 respectively. Each converter comprises semiconductor valves in a bridge connection known per se, such as, for example, a 2-level or a 3-level converter bridge. The semiconductor valves comprise branches of gate turn on/turn off semiconductor elements, for example power transistors of so-called IGBT-type, and diodes in anti-parallel connection with these elements.
Each converter is via a phase reactor 2 and transformer 1 connected to a three-phase alternating current electric power network N1 and N2, respectively. Although not shown in the figure, the converters may be connected to the three-phase network directly without transformers. Under certain circumstances the phase reactor is replaced by a transformer. Filter equipment 3 are connected in shunt connection at connection points between the phase inductors and the three-phase networks.
The first converter station STN1 comprises control equipment CTRL1 for generation switching control pulse FP1, which comprises turn on/turn off orders to the semiconductor valves according to a predetermined pulse width modulation pattern. The inputs to the converter control, in addition to reference orders such as DC voltage or active power and AC voltage or reactive power, comprises measured DC voltage Ud1, 3-phase AC current I1 and 3-phase AC voltage UL1. The inputs to the converter may also include measured 3-phase current in transformer It1 and 3-phase voltage at primary side of transformer UN1. The DC-voltage across the capacitor equipment C1 is designated Ud1 and is sensed with only symbolically shown sensing device M11. Similarly, signals I1, UL1, It1 and UN1 are sensed with sensing devices M12, M13, M14 and M15 respectively.
The second converter station STN2 comprises control equipment CTRL2, which is similar to the control equipment CTRL1, for generation switching control pulse FP2. The inputs to the converter control of STN2 are similar to those to the converter control of STN1.
The converter stations may operate in four different modes, one of dc-voltage control and active power control and one of ac-voltage control and reactive power control. Usually, one of the converter stations, for example the first one, operates under DC-voltage control for voltage control of the direct current link, whereas the second converter station operates under active power control and under AC-voltage or reactive power control. The operation modes are set either manually by an operator, or, under certain conditions, automatically by a not shown sequential control system.
From U.S. Pat. No. 6,400,585 a control system for a voltage source converter in an HVDC transmission system is previously known.
A previously known control equipment is shown in FIG. 2. The two control equipments CTRL1 and CTRL2 normally present in a HVDC transmission system are represented by common control equipment CTRL in FIG. 2A. Thus, the indices 1 and 2 are omitted for sake of simplicity.
An outer active/reactive power control loop 4 generates the reference values of converter current in dq-components which are the inputs to an inner current control loop 5. Although not shown in the figure, there are four sub-control loops. As an example, FIG. 2B shows the structure of an active power control loop ACPL. The DC voltage, reactive power and AC voltage control loop can be built up in a similar way. The active power control loop is used to control either the active power to/from the converter or the DC voltage. In the embodiment shown the active power control loop comprises an active power calculation unit 8, a signal processing or filtering unit 9, a comparator and a regulator 10. For instance, if the active power should be controlled, the control loop has reference Pref and measured active power P as inputs and results in output d-component current reference (iv—refd) via a regulator 10 with for example a proportional/integrating characteristics. The selection of active power control or DC voltage is determined by input signal UDC13 CTRLmod. The reactive power control loop, which results in q-component current reference (iv—refq), can be used to control either the reactive power to/from the converter or the AC voltage amplitude. The selection of reactive power control or AC voltage is determined by input signal UAC_CTRLmod.
The inner current control loop 5 tracks the reference values of converter current and generates the voltage reference for the converter. In order to have control on direct current quantities instead of three phase alternative current quantities, the converter current control system operates in a conventional way with three phase units (voltages and currents of the alternating current network) transformed into and expressed in a rotating two-phase dq-reference plane, arrived at via a transformation 6 to a stationary two-phase αβ-reference plane, and the transformation is realized with techniques known per se via signal ξ, which is the output of Phase-Locked Loop PLL. The signal ξ represents an electrical angle linearly increasing with time with a time rate proportional to the actual frequency of the alternating current network, and it is locked to and in phase with the phase position of the bus voltage of the alternating current network.
The inner current control loop 5, as shown in FIG. 2A, is supplied with the current reference Īv-refdq, which is the measured current I1 in FIG. 1 in dq-components Īvdq, and the bus voltage UL transformed to the dq-reference plane ŪLdq. The inner current controller 5 outputs in dependence thereon an output signal designated Ūv-refdq, which is the voltage reference vector for the bridge voltage of the converter in the dq-reference plane.
The current reference values Iv-refd and Iv-refq may be limited in accordance with specified operating conditions for the transmission system before further processing. Such limitation means, which may be implemented in known ways per se, are not treated in this context.
The reference transformation 6 in FIG. 2A transfers the converter reference voltage Ūv-refdq in a rotating dq-reference plane to the stationary plane having Ūv-refabc as components voltages reference values for the respective three phases of the alternating current system.
The voltage reference vector Ūv-refabc is supplied to the pulse width modulation unit 7 that generates in dependence thereon a train FPa, FPb, and FPc of turn on/turn off orders according to a predetermined PWM pattern supplied to the semiconductor valves. In according to prior art, the predetermined PWM is a carrier based PWM such as sinusoidal PWM (SPWM), or sinusoidal PWM including 3rd harmonic modulation (3PWM).
It is known in a feedback system that a conflict between the response speed and stability makes the design of a control system difficult. What should be noticed is that the design of control for a voltage source converter in power system applications such as HVDC or static var compensator is even more difficult, due to that there is not only harmonic stability but also high requirement on harmonic performance, in addition to low frequency stability. In the prior art control system as shown in FIG. 2, often the control speed has to compromise with the rigid steady-state harmonic performance. As a result of lower response speed, the cost for the components or apparatus in the main circuit may be increased, for instance, the valves may have to be dimensioned with larger voltage and current margin in to achieve good performance during a transient caused by short-circuit faults in the connected AC network.
Thus, there is a need for a HVDC system comprising VSC that offers a high effect for lesser investment cost.